General Structure:

// 1. Preprocessor Directives
#include <stdio.h>

// 2. Global Declarations (optional)

// 3. main() Function
int main() {
    // 4. Variable Declarations
    // 5. Code / Logic
    return 0;
}

Structure Explanation:

1. Preprocessor Directives: Includes header files using #include.
2. Global Declarations: Variables or functions declared outside main() and accessible throughout the program.
3. main() Function: The starting point of execution.
4. Variable Declarations: Declares variables before use.
5. Statements: Logic like printf(), loops, conditions, etc.
6. Return Statement: Ends the program and returns value to OS.

Sample Program:

#include <stdio.h>     // for printf()
#include <conio.h>     // for clrscr(), getch()

int main() {
    clrscr();                  // clears the output screen
    printf("Welcome to C!");   // displays message
    getch();                   // waits for a key press
    return 0;                  // ends program successfully
}

Explanation of Key Components:

• #include <stdio.h>: Required for input/output functions like printf().
• #include <conio.h>: (Console Input/Output) — Not part of standard C, used mainly in Turbo C.
  It provides:
  • clrscr(): Clears the output screen before displaying new output.
  • getch(): Waits for a key press so the output window doesn't close immediately.
• int main(): This is the entry point of the program. It's standard to use int so we can return a value to the operating system.
• return 0: Tells the OS that the program ran successfully. If you return any non-zero value, it usually indicates an error.
• void main(): Sometimes used, but int main() is preferred according to international C standards (like ANSI/ISO).

Summary:

• A C program must include main() function — it's where execution begins.
• int main() and return 0 are standard for proper completion.
• clrscr() and getch() are helpful for clean output, especially in Turbo C.
• conio.h is not available in all compilers — mainly for DOS-based environments.